Donor-acceptor-donor type organic spacer for regulating the quantum wells of Dion-Jacobson 2D perovskites

Two-dimensional (2D) metal halide perovskite system has attracted significant attentions due its tunable optoelectronic characteristics and outstanding environmental durability. However, quantum wells (QWs) of 2D perovskite severely restrain the charge transfer between organic/inorganic layers, resulting in the significantly lower photovoltaic performance of 2D perovskite solar cells (PSCs) in contrast to their 3D analogues. To intrinsically refine the QW structures of 2D perovskite for less quantum confinement, we innovatively design the D-A-D (D: donor; A: acceptor) type organic cation as 2D spacer. The push-pull effects between D, A unit endows the organic spacer intramolecular charge transfer characteristic, which effectively lower its bandgap. The resulting 2D perovskite demonstrates flattened energy landscape of the QWs, which suppresses the charge transfer barrier and elongates the carrier diffusion lengths in 2D perovskite film. As a result, optoelectronic properties of this new 2D perovskite resemble those of the 3D counterparts, delivering an impressive PCE of 18.6% for 2D (n = 4) PSCs. This is one of the highest reported efficiency for DJ-type 2D perovskite (n ≤ 4) to date. By rationally selecting other D and A unit, QW structures of 2D perovskites can be further adjusted, which opens huge possibilities for developing a new system of 2D perovskites.